Expectant mothers ask Dr. Donald Redelmeier about all sorts of pregnancy dangers from roller-coasters to bear attacks, but never about the one that counts: car crashes.

New research by Redelmeier’s team involving more than half a million Ontario women shows pregnancy is associated with a substantial risk of a serious traffic accident requiring emergency care. The increased risk was largest in the early second trimester.

Women in their second trimester were 42 per cent more at risk of a life-threatening crash while driving, compared to before they got pregnant, the study found. That amounts to an extra 75 crashes sending expectant mothers to hospital each month in Ontario alone, according to a statement released with the study.

“An effect of this magnitude equals about one crash for every 50 pregnant women,” said Redelmeier, a senior scientist at the Institute for Clinical Evaluative Sciences.

Pregnancy causes physiological changes — nausea, fatigue and insomnia — that can increase the risk of driver error, he said. Some women also complain of “baby brain,” momentary episodes of absentmindedness.

“Motor vehicle crashes are the leading cause of fetal death related to maternal trauma,” the researches write in the Canadian Medical Association Journal.

Car crashes involving pregnant women “are enormously stressful for all the clinicians involved,” Redelmeier said in an interview. Pregnancy makes emergency care and standard resuscitation more difficult. Even administering analgesia can be more complicated when it involves a pregnant woman.

“The mom will usually survive because of the extra blood and the extra padding she has, but the downstream consequences for the fetus we just don’t know, but it can’t possibly be good for you,” said Redelmeier, a professor of medicine at the University of Toronto and staff physician at Sunnybrook Health Sciences Centre, the country’s largest trauma unit.

If concussions are harmful in young children, “they’re also going to be a potential problem before you’re born.”

“We used to think that the surrounding amniotic fluid would be a perfect blanket for the fetus,” he said. “It’s just not true.”

For their study, the researchers identified every woman in Ontario who gave birth to a baby between 2006 and 2011. The researchers followed each woman for five years — four years before delivery and one year after delivery — for any emergency department visit due to a car crash, before, during or after pregnancy.

In all, the study involved a total of 507,262 women.

During the three years before pregnancy the women had 6,922 crashes (177 per month). During the second trimester, the women, as drivers, had 757 crashes (252 per month), a 42 per cent relative increase.

The first month of the second trimester was the most hazardous month, with a crash rate of 7.7 events per 1,000 individuals annually. The last month of the third trimester was the least hazardous (a crash rate of 2.4 events per 1,000 individuals annually).

The increased risk held regardless of the woman’s age, education, socioeconomic status, risky behaviours such as smoking and alcohol use, and other factors. The researchers also looked at season of the year, daylight, and roadway layout “and all sorts of other risk factors for a crash,” Redelmeier said.

The risk of a crash during the second trimester was similar in magnitude to the risk associated with sleep apnea, the authors report.

“It’s a combination of the fatigue, the insomnia and the stress, all building up as people try to get things done before the baby comes,” Redelmeier said.

But the second trimester is, paradoxically, often the time when women feel their best, even though the hormone changes are dramatic, he said. Women might feel over confident and drop their guard while driving.

“By the third trimester, they’re spending a great deal of time at home; there’s much more back pain and more taking it easy.”

The one-year period after delivery was the least hazardous year during the study period. “I refer to that as ‘staying at home,’” Redelmeier said. Even when women are in the car the baby is often with them, he said. “They’re very mindful to drive safety.

“What we’re suggesting is that they should be equally aware in the second trimester that the same baby is with them.”

Last week a woman asked whether she was at risk of a bear attack because of her hormones.

“And yet I’m never asked about roadway safety despite it being a much larger risk to adults in Canada,” Redelmeier said.

Prenatal care guidelines in Canada, the U.S. and Britain have been silent on roadway safety, “despite it being a substantial risk to mother and child,” said Redelmeier, who wants the guidelines revised. He said it’s a greater risk than for pre-eclampsia — a potentially life-threatening rise in blood pressure — or a blood clot during pregnancy. “It’s just because it’s not a unique risk that people don’t think about it.”

The study is not a reason for pregnant women to stop driving, Redelmeier stressed. “Almost all of these collisions could be prevented by a small change in driver behaviour.” He also stressed that driving shouldn’t be delegated to young husbands, who are even more dangerous behind the wheel.

“We think the message is a reminder of the importance of road safety — stop at stop signs, respect the speed limit, signal turns, minimize other distractions and always wear a seatbelt.”

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Car crashes a risk factor for pregnant women in second trimester, Canadian study finds

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